1. Field of the Invention
The present invention relates to an image processing technique and, more particularly, to an image processing technique for extracting feature points from a raster image, and converting the raster image into a vector image by executing functional approximation.
2. Description of the Related Art
In recent years, devices such as a display and printer having different resolutions tend to be used together, and cases wherein identical image information is displayed at different resolutions are increasing. When an image suited to a low-resolution device is to be displayed on a high-resolution device, image deterioration stands out since the resolution of the device cannot be utilized. For this reason, a vectorization technique for converting an image expressed in a raster format into a vector format, which is independent of a resolution, is required.
Such vectorization techniques have been conventionally examined. For example, a vectorization method for a binary image is disclosed in Japanese Patent Laid-Open No. 2005-310070. This method divides a contour of a binary image at points which meet the following conditions, and approximates divided sections using Bézier curves.
Angular point 1: a point where a change in vector that connects neighboring point sequences is acute-angled
Angular point 2: two corner points when a distance between continuous points is larger than a certain threshold
Inflection point: a point where a sign of an outer product of continuous vectors changes
As a sequence of coordinate points extraction processing, for example, a method of Japanese Patent No. 3026592 is disclosed. According to this method, inter-pixel vectors in horizontal and vertical directions are detected based on states of a pixel of interest and its neighboring pixels, and connection states of these inter-pixel vectors are discriminated, thereby extracting sequences of coordinate points of image data from the connection states of the inter-pixel vectors. This sequence of coordinate points extraction method can provide a significant width even to a thin line having a 1-pixel width by extracting sequences of coordinate points for respective pixel edges in place of a pixel central position.
A vectorization method which targets not only a binary image but also a line image or illustration image including several colors is disclosed in, for example, Japanese Patent Laid-Open No. 2006-031245. This method can vectorize a color image without generating any gap in principle by a processing sequence for extracting boundary lines common to color regions, and executing functional approximation. A sequence of coordinate points extraction method in this case extracts sequences of coordinate points by tracking lines having colors which have a difference equal to or larger than a threshold between neighboring pixels unlike Japanese Patent No. 3026592. Boundary lines common to color regions are calculated from the extracted sequences of coordinate points, and a common functional approximation result is applied upon outputting the color regions, thus attaining vectorization.
However, the conventional vectorization method defines extraction rules that refer to several sides before and after a point of interest upon extracting anchor points required for functional approximation from a raster image, and uniquely extracts anchor points. However, since a wide variety of sequence of coordinate points patterns are input, the extraction rules are complicated in proportion to the number of sides to be referred to, thus posing the following problems.                If the extraction rules suffer any consideration errors, for example, when an image shown in FIG. 1A is input, points to be extracted as anchor points often fail to be extracted, as shown in FIG. 1B.        Since a reduction of anchor points is not taken into consideration, anchor points are excessively extracted, as shown in FIG. 1C, resulting in a large data size.        In order to improve the precision, the number of extraction rules increases in proportion to the number of sides to be referred to, resulting in complicated processing.        